The response of retinal rod photoreceptors to light consists of a membrane hyperpolarization resulting from the decrease of a light-sensitive conductance in the outer segment1. According to the calcium hypothesis, this conductance is blocked by a rise in intracellular free Ca triggered by light2, a notion supported by the findings that an induced rise in internal Ca leads to blockage of the light-sensitive conductance3-9 and that light triggers a net Ca efflux from the outer segment via a Na-Ca exchanger, suggesting a rise in internal free Ca in the light10-13. We have now measured both Ca influx and efflux through the outer segment plasma membrane and find that, contrary to the calcium hypothesis, light seems to decrease rather than increase the free Ca concentration in the rod outer segment. This result implies that Ca does not mediate visual excitation but it probably has a role in light adaptation.
ASJC Scopus subject areas